1. Field of the Invention
The present invention relates to noise barriers and, in particular, to noise barriers having means to provide a high degree of sound absorption and sound transmission loss.
2. Description of the Invention Background
It is well known that serious problems are imposed on humans by virtue of excessive noise in our environment. Excessively high noise levels are frequently found adjacent to highways, airports and in diverse industrial and commercial settings. In view of such problems, various Federal and State authorities have enacted legislation limiting the noise levels permissible in numerous locales.
Heretofore, various structures have been proposed to reduce the noise transmitted from a noise generating source to a receiving location. One such structure is a wall placed between the noise source and the receiver. The wall is typically constructed from metallic, wooden or concrete sheets and serves to block sound from travelling directly from noise source to receiver. Case histories have shown that such walls simply do not function with sufficient effectiveness to reduce noise transmission to the desired level, basically, because attendant with the blocking of noise, was the reflection of noise from the wall. It will be readily appreciated that sound-reflecting walls were not satisfactory in the case where they were to be erected on both sides of a noise source, such as on a highway. In that situation, the reflection of noise from walls on both sides of the highway would cause the noise levels therebetween to be reach unacceptably high levels.
Another prior art form of noise barrier consists of a single sheet or wall having a noise absorptive material attached to the side thereof facing the noise source. Experience has shown that while the noise absorptive material in such a construction will absorb some of the noise impinging on it, a relatively high percentage of the noise will pass through the absorptive surface and is reflected back from the sheet. While an additional amount of noise will be absorbed when the reflected sounds pass through the absorptive material again, the level of unabsorbed sounds reflected back toward the noise source may still remain unacceptably high. As such, this construction of a noise barrier is also fraught with serious shortcomings.
In yet another form of prior art noise barrier, a panel is constructed having a front perforated sheet facing the noise source and a rear solid sheet placed in parallel relation to the first sheet so as to define a space therebetween. A sound absorbing material is disposed within the space. This form of noise barrier has also proven ineffective in absorbing noise to an acceptable degree. Noise entering the panel through the perforations is absorbed to some extent in the absorptive material; however, a significant amount of noise passes through the absorptive material and is reflected from the rear wall back into the absorptive material. Experience has shown that an unacceptable amount of noise passes through the absorptive material and exits the panel through the perforations in the front sheet. Accordingly, an the amount of noise not absorbed within the panel and returned to the noise source, where it may combine with the noise source may still be unacceptable.
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Still another prior art form of barrier comprises a combination heat and sound insulating barrier. In such a structure, a perforated front or noise facing sheet is provided as well as a first intermediate solid sheet with mineral wool disposed therebetween. A thin core of sound insulating material is provided against the opposite face of the first intermediate sheet. A second solid intermediate sheet is provided adjacent the opposite face of the sound insulating material. A layer of mineral wool felt is provided between the opposite face of the second intermediate sheet and a rear solid sheet. Applicants question the sound absorbing effectiveness of such a construction. It would appear that noise will not effectively pass into the sound absorbing material and the second layer of mineral wool due to the presence of the first and second solid intermediate sheets. Accordingly, the above-described construction of a noise barrier appears most costly and does not appear to effectively absorb a sufficient amount of noise.
The subject invention is directed toward an improved noise barrier which overcomes, among others, the above-discussed problems and provides a noise barrier which in addition to reducing sound transmission is effective to absorb a signicicant amount of noise while providing a cost-effective, easily installed and maintenance-free construction.